The invention relates to a record carrier having an information structure which comprises optically readable information areas arranged in information tracks, adjacent information track portions differing from each other in that they comprise information areas of a first phase depth and information areas of a second phase depth respectively. The invention also relates to apparatus for reading such a record carrier.
Such a record carrier and apparatus for reading it are described in the Applicants' Netherlands Patent Application No. 78 03517 corresponding to abandoned U.S. Application Ser. No. 419,640, filed Sept. 17, 1982. In the record carrier described therein the first phase depth is preferably approximately .pi.rad . and the second phase depth approximately 2.pi./3 rad.
When the information structure is scanned with a read beam, this beam is split into a zero-order subbeam and a plurality of higher order subbeams. The phase depth is defined as the difference between the phase of the zero order subbeam and the phase of one of the first order subbeams when the centre of the read spot formed on the information structure coincides with the centre of an information area. In said Netherlands Patent Application No. 78 03517 it is demonstrated that if the information areas in each of two adjacent information track portions have different phase depths, these track portions can be arranged more closely to each other than in the case where the information structure comprises information areas which all have the same phase depth. The information content of a record may then, for example, be doubled, without any significant increase in cross-talk between adjacent track portions.
However, the information track portions of different phase depths should then be read in different ways. The information track portions with the greater phase depths are read by determining the variation of the total intensity of the radiation received from the record carrier and passing through the pupil of the read objective. This is the so-called integral or central aperture read method. The information track portions with the smaller phase depth are read by determining the difference of the intensities in two tangentially different halves of the pupil of the read objective. This is the so-called differential read method.